Hip-sacroiliac joint-spine syndrome in total hip arthroplasty patients

This study is designed to compare the extent of sacroiliac joint (SIJ) degeneration at total hip arthroplasty (THA) for two pathologies: osteoarthritis of the hip (OA) and osteonecrosis of the femoral head (ON). We also assessed the prevalence of SIJ degeneration in patients with lumbar spondylolisthesis or degenerative scoliosis. A total of 138 hips from 138 patients (69 OA and 69 ON) were assessed in this study, including 66 hips affected by OA secondary to developmental dysplasia of the hip. The degenerative changes in the SIJ and lumbar spine were evaluated prior to THA using radiographs and computed tomography (CT) scans, showing 9 instances of spondylolisthesis and 38 of degenerative scoliosis. The OA group exhibited longer duration from onset to surgery than the ON group. The OA group also included more cases with significant pelvic obliquity (3 degrees or more) and with significant increases in SIJ sclerosis and irregularities. Patients with lumbar spondylolisthesis or degenerative scoliosis were significantly more likely to have SIJ irregularities. The prevalence of SIJ degeneration was higher in cases of THA for OA than for ON. This study also suggests the possibility of Hip-SIJ-Spine syndrome in THA patients with OA.


Methods
The subjects of this study were patients who underwent THA for the treatment of OA or ON.A total of 93 patients (100 hips) who received THA for OA between June 2016 and December 2016 were enrolled.Since there was a possibility of changes in findings after unilateral surgery, seven cases of bilateral procedures were included only for analysis of the first operated side.An additional 24 patients (24 hips) were excluded because of inadequate CT confirmation of SIJ, so that 69 patients (69 hips) were included in analysis for the OA group.Because the annual number of cases of ON is relatively small, the treatment period for ON was extended from January 2010 to October 2022, and 83 patients (109 hips) were enrolled.Of those, 26 bilateral cases were counted on one side only.Seven patients (7 hips) were excluded because the patient had already undergone THA on one side, and an additional seven patients (7 hips) were excluded because of inadequate confirmation of the SIJ by CT or because of significant collapse of the necrotic area from advanced OA.This resulted in 69 cases (69 hips) that were included in analysis for the ON group.
Patient demographics are shown in Table 1.There were no significant differences in age, height, weight and BMI between the groups.However, due to a higher prevalence of OA resulting from developmental dysplasia of the hip (DDH), female patients made up a significantly higher portion of the OA group (87%) than of the ON group (55%).In the OA group, only 3 hips were classified as primary OA, while 66 hips (96%) were classified as secondary OA from DDH. Crowe classification Group I accounted for 61 hips (92%), indicating a relatively low degree of hip dislocation in this group.In the ON group, 38 hips (55%) were classified as Group 4, indicating narrowing of the joint space but not reaching terminal OA.
Preoperative leg length discrepancy was measured by assessing the difference in Spina Malleolar Distance between the left and right sides before surgery, and was classified as an absolute value of ≤ 5 mm or ≥ 6 mm.A difference of ≥ 6 mm was defined as leg length discrepancy.

Radiographic and CT analysis
The following items were investigated using imaging: I. Preoperative anteroposterior pelvic radiographs www.nature.com/scientificreports/ 1. Sclerosis: Presence of sclerosis adjacent to either the sacral or iliac side of the SIJ was classified as sclerosis present.2. Vacuum phenomenon: Presence of CT attenuation similar to air in the joint space of the SIJ was classified as vacuum phenomenon present.3. Discrepancy in joint space width: Confirmation of asymmetry in the width of the SIJ space was classified as discrepancy in joint space width.4. Irregularities: Slight narrowing of the SIJ space, non-parallel joint surfaces of the sacral or iliac side, or erosive changes in a portion of the joint surface were classified as irregularities present. 5. Discrepancy in iliac wing opening angle: Measurement of the angle formed by drawing a perpendicular line from the midline sacral ridge to the vertebral body at the second sacrum level and drawing a line from the midpoint of the width of the most anterior part of the iliac wing visible on the CT slice to the sacral ridge.A difference in absolute values of 3 degrees or more between the left and right sides was classified as iliac wing opening angle discrepancy present.

Statistical analysis
Statistical analysis was performed using the chi-square test or Fisher's exact test by Microsoft Excel.A p-value of 0.05 or less was considered statistically significant.
All methods in this study were performed in accordance with the relevant guidelines and regulations, and approval was granted by the Ethics Committee of Kanazawa Medical University.

Ethical approval
Approval was granted by the Ethics Committee of Kanazawa Medical University.

Consent to participate
Written informed consent was obtained from the all patients.

Results
The OA group differed significantly from the ON group in several parameters.Significantly more prevalent in the OA group were duration of illness, angle of pelvic outward tilt, instances of pelvic outward tilt of 3 degrees or more, and sclerotic and irregular images in the SIJ (Table 2).No significant intergroup differences were noted in the frequency of lumbar spondylolisthesis and scoliosis.However, the presence of lumbar spondylolisthesis and of 5 degrees or more of scoliosis were significantly associated with a higher prevalence of irregular SIJ images (P = 0.027) and (P = 0.00373), respectively (Table 3).Further analysis revealed that the presence of SIJ irregularities was significantly associated with asymmetry in the SIJ joint space in the entire group (P < 0.0001).No significant differences were observed in other parameters.
Case study 1 (Fig. 1a-c): A 58-year-old female presented with bilateral hip OA.She had been experiencing bilateral hip pain for the past 10 years.The pain gradually worsened, leading to limited range of motion, and she underwent left THA.Subsequently, a right THA was also performed.www.nature.com/scientificreports/Case study 2 (Fig. 2a-c): A 51-year-old male presented with left hip pain continuing for the past 3 months and was diagnosed with stage 3 ON on the left side.Due to severe pain, he underwent left THA.

Discussion
The results of this study suggested that THA was associated with significantly more degenerative change in the SIJ of the OA group than the ON group.In addition, the presence of irregularities in the SIJ was significantly associated with greater asymmetry in the SIJ space, indicating that excessive load on the SIJ due to hip OA may have led to degenerative changes 24,25 .This study showed little correlation with leg length discrepancy, suggesting that SIJ degeneration may progress even in the absence of conditions such as DDH-related leg shortening.However, it is also possible that DDH itself increases stress on the SIJ.Toyohara et al. analyzed SIJ stress using finite element models based on CT images before and after periacetabular osteotomy in four DDH patients 28 .They reported that stress on the SIJ and the posterior sacroiliac ligament, which is commonly observed in preoperative dysplastic hips, often decreased after surgery, indicating that excessive load on the SIJ was reduced after formation of a near-normal acetabulum.This suggests that the morphology of DDH itself may increase the load on the SIJ.
A significant association was noted between SIJ joint irregularities and the presence of more than 25% of lumbar spondylolisthesis or scoliosis of 5° or more, indicating a higher prevalence of SIJ degeneration in the patient group that underwent THA and had degenerative spinal diseases.Kwon et al. reported a higher prevalence of SIJ degeneration in patients with spinopelvic imbalance compared to those with lumbar spinal canal stenosis (LSCS) 29 , and Chen has stated that SIJ degeneration was more common in patients with spondylolisthesis 30 .Although a systematic review has shown no consensus regarding SIJ degeneration after lumbar spinal fixation 31 , there has been some suggestion that the SIJ degeneration described in the previous reports 24,25,30 might have been caused by spinal disease or hip OA.However, our study showed SIJ degeneration in some cases of Hip-Spine syndrome, in which spinal degeneration is associated with hip OA.Although further investigation is needed in a larger number of clinical cases, of course, with careful consideration of clinical symptoms, the new concept of "Hip-SIJ-Spine syndrome" should be focused on patients with Hip-Spine syndrome.
This study indicated that long-term degeneration of the hip joint has a major impact on the SIJ, making it more susceptible to Hip-SIJ syndrome.And patients with the condition termed "Hip-Spine syndrome" may show a higher prevalence of SIJ degeneration, suggesting the existence of what we have designated as "Hip-SIJ-Spine syndrome".
This study did not make use of the SIJ degeneration score developed by Backlund et al. or Eno's classification 32,33 .This was because previous reports on the relationship between hip OA and SIJ degeneration 25 showed no significant difference in scores between the OA and control groups, and data on osteophyte levels were contradictory.In the present study, SIJ joint irregularities, which were not included in the SIJ degeneration score, were found to be associated with hip OA.SIJ joint irregularities are often observed on CT scans as early changes and may be interpreted as the first imaging of SIJ degeneration 27 .Considering that SIJ instability can induce symptoms 19 , these changes may also represent the first imaging of instability.Further investigation is needed in the future.
This study had several limitations.First, observations were limited to cases that underwent THA, and it is unclear whether similar occurrences of Hip-SIJ-Spine syndrome are more likely in cases of DDH or femoroacetabular impingement.Second, no data is currently available on whether SIJ degeneration is similarly likely to occur in simple Hip-Spine syndrome where pathology extends from the lumbar spine to the hip.Third, clinical symptoms were not confirmed, so it is unclear whether the concept of Hip-SIJ-Spine syndrome is clinically www.nature.com/scientificreports/relevant or clearly expressed.Fourth, cases of DDH-related hip OA predominated in this study, and we do not know whether the same pathology would occur in cases of primary hip OA.Fifth, the sample size was small.Diagnosing SIJ dysfunction was not easy in the past, but it has been facilitated by the SIJ scoring system developed by Kurosawa et al. 18 and based on physical examination and pain region.The SIJ score has a total of 9 possible points, including 2 points for groin pain.In this context, for physicians working with OA patients who require THA, our findings may be useful clinically as a warning that groin pain should be differentiated from SIJ if the spine also has degenerative disease.Further research is warranted.

Figure 1 .
Figure 1.Hip OA secondary to DDH.(a) Pre-operative x-ray.The patient had bilateral terminal-stage OA of the hip joint.The SIJ space differed visibly between right and left sides.(b) Prior to left-side surgery, the patient was diagnosed with a 25% slip of the fourth lumbar vertebra indicative of spondylolisthesis.Scoliosis with a Cobb angle of 7 degrees was also observed.(c) CT imaging showed SIJ irregularities and the presence of vacuum phenomena (yellow arrows).OA: osteoarthritis, DDH: developmental dysplasia of the hip, SIJ: sacroiliac joint, CT: computed tomography.

Figure 2 .
Figure 2. Alcohol-induced ON.(a) The sclerotic boundary was visible (white triangles), with bone collapse on the left femoral head (yellow arrows).(b) X-ray images showed no lumbar spondylolisthesis except for slight scoliosis.(c) CT images showed no abnormalities, including SIJ irregularities.ON: osteonecrosis of the femoral head, SIJ: sacroiliac joint, CT: computed tomography.

Table 1 .
Demographic data for all patients.OA osteoarthritis of the hip joint, ON osteonecrosis of the femoral head, BMI body mass index.*Values are mean and range, NA not applicable, NS not significant.

Table 2 .
Comparison of OA and ON groups.SIJ sacro-iliac joint, OA osteoarthritis of the hip joint, ON osteonecrosis of the femoral head.# Values are median and range.*Values are mean and range.Statistical analysis for continuous values was performed using a t-test.A chi-square test was conducted for the other variables.

Table 3 .
Relationship between lumbar spine degeneration and SIJ degeneration.SIJ sacro-iliac joint, Fisher's exact test was conducted for all analyses.